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Tuesday, August 31, 2010

Control Your Depth of Field

Try the Focus Slice Auto-Align technique to get tack-sharpness from near to far

Labels: How-To

This Article Features Photo Zoom

Figure 4: Chromatic aberration can be adjusted in Adobe Camera Raw (or Lightroom) using the Lens Corrections tab. Most red/cyan aberration can be greatly improved using the slider as shown, usually in a negative direction.
Helicon Focus
Helicon Focus was initially designed for microscopists. The idea is to take photographs at many focusing points through a scene to produce a final blended image that’s sharp throughout. While the technique works well, and it’s also useful for the landscape photographer, the technique has some inherent limitations. Helicon Focus is an interpolation technique that can produce some problems such as “ringing artifacts.” These are rings around some structures, especially sharp-edged, bright subjects such as trillium flowers.

Also, if there’s movement in the image (water, moving clouds), serious image degradation occurs. The Helicon Focus designers recognized both problems and have designed a Retouching tool to deal with them; however, I’ve had limited success with retouching.

Focus Slice Auto-Align
Another technique for precisely controlling sharp depth of field uses “focus slicing” as you would do for Helicon Focus, but the images are combined using Auto-Align in Photoshop. The Focus Slice Auto-Align technique has no inherent limitations because the procedure isn’t interpolative, and the focus slices are layered on each other with the sharp zones of each slice “painted” onto the final composite image.

Figure 5: The Foreground slice was opened first with the Horizon slice placed next to the Foreground slice by clicking on Arrange, then Tile in the Auto-Align function of Photoshop CS4.
The limitations that exist are procedural, and there also are some physical limitations inherent in SLR lenses, especially wide-angle lenses. The procedural limitations are: 1) the technique is labor-intensive in contrast to Helicon Focus, which is extremely fast, and; 2) each focused slice must slightly overlap the next slice in focused sharpness. If the focus slices don’t overlap in sharpness, there will be zones of “lack of sharpness” between slices that can’t be corrected. Overlapping zones of focus also are needed for Helicon Focus.

The lens limitations are concerned with diffraction problems caused by small-aperture diameters at larger ƒ-stops in SLR lenses. ƒ-stop is defined by the mathematical equation: ƒ-stop = lens focal length divided by aperture diameter.

For a wide-angle lens such as a 24mm at ƒ/22, the aperture diameter is approximately 1mm. That’s a small opening. So when the lens focuses an image and it passes through an aperture at ƒ/22, there’s considerable diffraction as the image passes through the small aperture and is focused on the sensor (or film). At apertures ƒ/16 and beyond, there’s a progressive degradation in image quality, so SLR lenses should be used at “sweet spots,” ƒ-stops around ƒ/8 to ƒ/11, when possible. Of course, that means a reduction in depth of field, so to use focus-slicing techniques, it’s best to use several focus slices to ensure sharpness throughout the final composite.


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